Polarplot in dB.

Hi, I am trying to plot a certain function (traveling wave) using polarplot(). However, the result isn't even close to that presented in the attachment, which is supposed to be the polar plot of the very same function! I have been trying for two days now to figure out what might be causing this visual discrepancy, to no avail. Below is my calling of polarplot:

theta=transpose(-1:0.01:1);
MagE=(cot(theta*pi/2).^2).*(sin(5*pi*(cos(theta*pi)-1))).^2;
polarplot(theta*pi,MagE,'Linewidth',1,'color',[.21 .81 .94]);

I also tried changing MagE to db(MagE), as the plot in the attachment appears to be in dB, yet it didn't do the trick. I hope someone here might be able to suggest a solution. I'd appreciate any assistance, advice, promptly if possible. Thanks

in advance.

Answer 1

[Honglei Chen]

You need to be careful about dB values as they might be negative. So you need to preprocess it to make it all positive before plotting and then fix the labeling. Here is an example

MagEdB = 10*log10(MagE);
MagEdB = MagEdB - max(MagEdB);
MagEdB(MagEdB<-40) = -40;
h = polarplot(theta*pi,MagEdB+40,'Linewidth',1,'color',[.21 .81 .94]);
haxes = get(h,'Parent');
haxes.RTickLabel = {'-40','-30','-20','-10','0'};

HTH

[Yuval]

Thank you so much! One question though - how do I get the lobes to meet at zero? I want the drawing to start from the origin. I tried to applying your code to another wave function and what I got was this (which as you can see doesn't begin at 0):

[Honglei Chen]

If you are talking about the gap, you just need to make sure you have values at 0 degrees and you may need to have it at the beginning and the end of the array to close the gap. As to the radius, there is no guarantee that you can start from the origin because that depends on the value at that angle.

HTH

[Honglei Chen]

BTW, another possibility is your data. If you look at your original MagE, the value in the middle is NaN, it's probably because you have a 0/0, but there is a valid limit value for that point. You may need to condition your data first.

HTH

[Honglei Chen]

Looking at your equation, when theta is 0, you know that the function should give you the value of 0 when it approaches 0. However, because the way you construct your function, when MATLAB evaluates it, it notices that it is inf*0, so it gives you an NaN (Think for example the case of sin(x)/x). Therefore, you need to identify those cases yourself before you plot so it does not try to plot NaN, which is a gap.

For example, in your original, there is a gap at theta==0. If I do the adjustment before plotting, then it works just fine.

MagE(theta==0) = 0;  % <- preconditioning
MagEdB = 10*log10(MagE);
MagEdB = MagEdB - max(MagEdB);
MagEdB(MagEdB<-40) = -40;
h = polarplot(theta*pi,MagEdB+40,'Linewidth',1,'color',[.21 .81 .94]);
haxes = get(h,'Parent');
haxes.RTickLabel = {'-40','-30','-20','-10','0'};

HTH